The invention relates to a process for controlled production of supported zeolite membranes, to the membranes obtained and to their use in separation processes.
Zeolites have the principal advantages of having a crystalline structure and a defined pore size, of having modifiable surface properties (in particular in terms of hydrophilic nature/organophilic nature and acidity) and linked to the chemical composition of the framework. The particular topology of zeolites and their cation exchange properties means that they can be used for applications of separation by selective adsorption or for catalytic reactions. For a given structural type, connected to the crystalline family to which each zeolite belongs (these structure types have been described in the work by Meier, W. M., Olson, D. H., in xe2x80x9cAtlas of Zeolite Structure Typesxe2x80x9d (1992), Butterworth-Heinemann, Ed.), separation of molecules present in a mixture can take place by selective adsorption and/or size exclusion in particular. However, separation on a powdered zeolite is a discontinuous process. In contrast, a zeolite membrane offers the possibility of separating molecules by a continuous process, which may be particularly advantageous from a technological and economical viewpoint.
A variety of processes for producing zeolite membranes have already been described. The hydrothermal route using porous supports has the advantage of stabilising the zeolite crystals in a porous matrix (alumina, stainless steel, for example) and at the surface thereof. European patent application EP-A-0 778 075 describes a process for producing zeolite membranes supported by porous glass. United States patent U.S. Pat. No. 5,429,743 and International patent application WO-A-95/29751 describe protocols for producing composite membranes supported by an inorganic macroporous matrix. Reference can also be made to documents U.S. Pat. No. 4,099,692, WO-A-93/19840, U.S. Pat. No. 5,567,664 and WO-A-96/01683. International patent application WO-A-00/33948 describes a process for producing composite zeolite membranes supported on tubular solids that are optionally multi-channelled. Such composite zeolite-based membrane materials are formed from a zeolitic phase deposited on a support.
Such preparations of membrane materials are carried out by isothermal heat treatment of a mixture containing precursors of the zeolitic phase, which is active for separation. Crystallisation is carried out at a fixed temperature maintained during that step of the synthesis (isothermal treatment). The zeolite crystallisation step can also be repeated a number of times. The synthesis is then reproduced after optionally cooling the material to ambient temperature, washing and drying of said material. The operations are identical and enable successive layers and/or zeolite crystals to be deposited that fill the interparticular spaces. In such cases, the preparation period is considerably prolonged. That mode of multi-step synthesis also encourages the production of thick layers of zeolites, which can crack when calcining the membrane (Vroon, Z. A. E. P., Keizer, K., Burggraaf, A. J., Verweij, H., J. Membr. Sci. 144 (1998) 65-76). Further, increasing the thickness can considerably limit the transfer of material through the membrane during the separation operation, thus reducing the technical and economic advantage of the membrane separation operation, due to a reduction in the productivity of the separation step. Further, this mode of multi-step synthesis requires a large quantity of precursors for the zeolitic phase, which considerably increases the cost of the starting materials and the precursors used. It also has the disadvantage of prolonging the period for producing the membrane material and augmenting the operating cost of the separation operation.
Apart from the membrane material preparation field, zeolite crystals (powdered material) have been obtained during polythermal preparations, carried out at different temperatures as the reaction progresses, in the absence of a support. U.S. Pat. No. 5,089,243 describes preparing alumino-silicate powders using a two-step crystallisation process in the absence of an organic agent. The first step is carried out at 240xc2x0 C. to 325xc2x0 C. for 1 to 20 minutes. The second step is carried out between 120xc2x0 C. and 225xc2x0 C. for 1 to 100 hours. Recently, non-supported zeolite crystals have been obtained by raising the temperature during synthesis (Li, Q., Creaser, D., Sterte, J. Microporous and Mesoporous Mater. 31 (1999) 141-150). A first step is carried out at 60xc2x0 C. or 80xc2x0 C. and enables the number and density of the crystals to be controlled. Raising the temperature to 100xc2x0 C. encourages crystallite growth. In those two types of preparation, the materials obtained are powders, in the form of divided solids, and in no case constitute membrane materials with a solid continuous layer for use in separation.
One of the difficulties of preparing zeolite-based membranes resides in controlling zeolite crystallisation to obtain zeolite crystals that are properly bound to the support, principally localised in the pores of the support, forming a continuous composite zeolite/support layer (obtained by obstructing the voids in the support with crystals of the zeolitic phase) and preferably sufficiently fine to limit the resistance to transfer through the membrane material. The majority localisation of the zeolitic phase in the pores of the support endows it with very good thermal resistance and mechanical resistance of the membrane material. However, it is not excluded that a minor portion of the zeolitic phase is localised on the external surface of the support. One of the essential aims of the present invention is to provide a method for controlled production of supported zeolite membranes in which the zeolitic phase has the characteristics described above. The zeolitic membranes obtained by the process of the invention have better separating power than membranes synthesised in accordance with prior art methods. They also have very high structural integrity, i.e., an absence of defects in the structure of the zeolitic phase and an absence of interparticular spaces, i.e., the voids present between the zeolite crystals.
The present invention concerns a process for preparing a supported zeolite membrane constituted by a composite continuous zeolite/support layer of controlled thickness, wherein the zeolite crystals are principally localised in the pores of a porous support and optionally on the external surface thereof. This preparation method comprises at least the formation of a precursor gel of said zeolite, bringing said gel into contact with said support and crystallising the zeolite from said gel. The preparation process of the invention is characterized in that said zeolite is crystallised by carrying out a non-isothermal thermal programme comprising at least three steps in succession constituted by a first constant temperature stage carried out at a temperature in the range 50xc2x0 C. to 300xc2x0 C. followed by cooling to a temperature of strictly less than 50xc2x0 C. followed by a second constant temperature stage carried out at a temperature in the range 50xc2x0 C. to 300xc2x0 C. This preparation process of the invention can produce high performance materials for separation in a single step (one-pot crystallisation).
The first constant temperature stage is preferably carried out at a temperature in the range 80xc2x0 C. to 220xc2x0 C. This first constant temperature stage is maintained for a period in the range 1 hour to 15 days, preferably in the range 3 hours to 72 hours.
The reduction in temperature from the first constant temperature stage is such that the mixture is cooled to a temperature that is strictly less than 50xc2x0 C., preferably less than 40xc2x0 C. and is maintained at that temperature for a period of 1 minute to 72 hours, preferably in the range 20 minutes to 9 hours, more preferably 30 minutes to 5 hours.
The second constant temperature stage is preferably carried out at a temperature in the range 80xc2x0 C. to 220xc2x0 C., more preferably carried out at a temperature equal to that of the first constant temperature stage. This second constant temperature stage is maintained for a period in the range 1 hour to 15 days, preferably in the range 3 hours to 72 hours.
The process for producing the zeolitic membrane of the invention comprises at least (a) forming a gel (or a solution) constituted by a mixture of precursors for the zeolite containing the agents necessary for the formation of said gel and bringing it into contact with the porous support, (b) crystallising the zeolite from said gel using a non isothermal thermal programme as defined above and including, in succession, at least one constant temperature stage at a temperature in the range 50xc2x0 C. to 300xc2x0 C., cooling to a temperature strictly less than 50xc2x0 C. followed by a second constant temperature stage at a temperature in the range 50xc2x0 C. 300xc2x0 C. and finally (c) eliminating residual agents.
This process results in a supported zeolitic membrane constituted by a composite zeolite/support layer, which is continuous and thin, wherein the zeolitic phase formed from zeolite crystals is principally localised in the pores of a porous support and optionally on the external surface of said support, to allow good adhesion of the zeolite to the support and increased thermal and mechanical resistance of the composite material. Further, the thickness of this layer and the crystal size and morphology are controlled. The thickness is controlled by the quantity of gel, and thus in particular by the quantity of zeolite precursor agents used in the preparation, incorporated into the porous matrix. The size and morphology depend on the relative proportions of the different constituent reactants of the precursor gel of the zeolite. As a result, this process can produce thin layers that are particularly suitable for use in separation.
More precisely, in the process of the invention, the support is constituted by a porous material wherein the total pore volume fraction is more than 5%, preferably more than 40%, and wherein the pore diameter is in the range 4 nanometers to 100 micrometers, preferably in the range 4 nanometers to 10 micrometers. The support is constituted by a material that is preferably inorganic, but it may be organic, metallic or mixed in nature. A ceramic support based on alumina and/or zirconia and/or titanium oxide is a suitable example. Other materials of the following nature may also be suitable: carbon, silica (aerogel, porous silica), zeolites, clays, glass (sintered glass, Vycor(copyright) glass, glass microfibres), polymers, metals (stainless steel, silver). The use of an alumina support of the allotropic alpha or gamma type (in particular an alpha alumina) is preferred. This support can optionally be constituted by a plurality of layers of varying porosity. The support can have any geometry, for example tubular, coiled, flat, in the form of a disk, a sheet or fibres.
The porous support is brought into contact with a solution containing the agents necessary for the formation of the zeolite precursor gel. Said solution containing the hydrolysable framework sources is constituted by these sources, either pure or diluted. The hydrolysable framework elements are susceptible of producing tetrahedra of the TO4 type (where T represents an element selected from Si, Al, B, Ga, Ge and P) are preferably constituted by alkoxides in the case of silicon or aluminium and/or silicon tetrachloride, and/or aluminium trichloride and/or aluminium salts such as aluminium sulphate and/or sodium aluminate. Highly preferably, they are silicon and/or aluminium alkoxides. The solution preferably contains water and is optionally supplemented with a polar organic molecule and/or one of several aids to the reaction for forming the zeolite precursor gel and/or one or more aids to zeolite crystallisation.
Aids for the gel formation reaction are constituted by mineral and/or organic acids or bases that act as catalysts. Hydrochloric and hydrofluoric acid, sodium hydroxide and ammonia are suitable examples.
Aids for the zeolite crystallisation reaction are constituted by acids or bases and/or salts of minerals and/or organic compounds and/or undissociated molecules essentially acting as mobilising agents and/or structuring agents (structure-promoting agents/templates) and ensuring the neutrality of the charge on the framework as counter-ions. Hydroxide or fluoride ions are the principal mobilising agents and are introduced into the preparation medium, for example in the form of sodium hydroxide, organic hydroxides and hydrofluoric acid. These mobilising agents allow the precursors to dissolve. A variety of mineral or organic templates may be suitable: hydrated cations (sodium or potassium ions), ion pairs (ammonium or phosphonium ions and the corresponding anions) or neutral molecules (amines, alcohols or ethers). The most frequently used crystallisation aids are tetrapropylammonium hydroxide or bromide or a mixture of the two, sodium and potassium hydroxides, ammonia, hydrofluoric acid and ethers such as crown ethers and cryptands.
The first gel formation step consists of drying the porous support, cooling it to ambient temperature and impregnating it with the solution containing the sources of the hydrolysable framework elements. The support can be dried at a temperature in the range 40xc2x0 C. to 600xc2x0 C., for a period in the range 1 minute to 48 hours. It is preferably carried out between 60xc2x0 C. and 80xc2x0 C. overnight. Impregnation preferably consists of immersing the support in the solution. However, other methods can be used, for example depositing the solution on the surface of the porous support. Impregnation can be carried out at a temperature in the range xe2x88x9220xc2x0 C. to 200xc2x0 C., for a period in the range from 1 minute to 72 hours, and at a pressure in the range 10xe2x88x925 atmospheres to 1 atmosphere. As an example, impregnation can be carried out at ambient temperature for 30 minutes and at a pressure of 2xc3x9710xe2x88x922 atmospheres.
The zeolite is then crystallised hydrothermally. To this end, the reaction mixture resulting from the first step for gel formation and contact with the porous support is placed in a polytetrafluoroethylene (PTFE) lined autoclave as described, for example, in EP-A-0 778 076, and the mixture is then subjected to temperature programming under autogenous pressure. The first zeolitic stage crystallisation phase is carried out at a temperature in the range 50xc2x0 C. to 300xc2x0 C., preferably in the range 80xc2x0 C. to 220xc2x0 C. and for a period in the range 1 hour to 15 days, preferably in the range 3 hours to 72 hours. The to mixture is cooled in a second stage to a temperature that is strictly less than 50xc2x0 C., preferably less than 40xc2x0 C., and highly preferably, it is cooled to ambient temperature and maintained at that temperature for a period of 1 minute to 72 hours, preferably 20 minutes to 9 hours and more preferably 30 minutes to 5 hours. The system is then heated to a higher temperature between 50xc2x0 C. and 300xc2x0 C., preferably between 80xc2x0 C. and 220xc2x0 C. and more preferably to the temperature of the first stage for a period in the range 1 hour to 15 days, preferably in the range 3 hours to 72 hours.
The membrane formed is cooled to ambient temperature for washing to at least partially eliminate the aids to the gel formation reaction and/or those for the zeolite crystallisation reaction. Washing is preferably carried out with distilled water. The membrane is then dried, preferably in the range 60xc2x0 C. to 80xc2x0 C., for a period in the range 2 hours to 24 hours, and cooled to ambient temperature.
The preceding gel formation reactions followed by zeolite crystallisation using the non isothermal temperature programme of the invention, then cooling the membrane, washing then drying the material, can be repeated a number of times.
The membrane is then calcined by steadily increasing the temperature to a temperature in the range 300xc2x0 C. to 800xc2x0 C., preferably in the range 350xc2x0 C. to 600xc2x0 C. This temperature rise can be carried out continuously or in constant temperature stages for a period in the range 5 hours to 50 hours. The calcining temperature is then maintained at that value for a period in the range 10 minutes to 24 hours. The membrane is then gradually cooled to ambient temperature. Cooling can be carried out continuously or in constant temperature stages for a period in the range 1 hour to 24 hours.
The high quality of the membranes obtained from the process of the invention, as will be shown in the following examples, has no connection with the rates of temperature rise (temperature gradient) when carrying out the non isothermal thermal programme.
This process can be applied to all zeolites, i.e., all crystalline solids characterized by a structure comprising a three-dimensional framework resulting from a concatenation of TO4 type tetrahedra (where T=Si, Al, B, Ga, Ge and/or P), each oxygen atom being common to two tetrahedra, and channels and cavities with molecular dimensions. Suitable examples are structure types FAU, GME, MOR, OFF, MFI, MEL, FER, LTA and CHA, using the IUPAC nomenclature (Meier, W. M., Olson, D. H., in xe2x80x9cAtlas of Zeolite Structure Typesxe2x80x9d (1992), Butterworth-Heinemann, Ed.).
This process can also be applied to any solid resulting from a concatenation of TO6 type octahedra (where T=Ti, Mn and/or Mo), for example titanosilicates, or to mesoporous solids of the MCM-41 and MTS (Micelle Templated Solids) type. In the latter case, micellar organic agents are used as crystallisation aids.
The thickness of the continuous zeolite layer is controlled by the quantity of gel incorporated. This thickness is in the range 0.5 to 100 micrometers, preferably in the range 1 to 50 micrometers. To limit the resistance to molecular transfer through the material during its application, the thickness is highly preferably in the range 1 to 15 micrometers. The crystal size and morphology depends on the relative proportions of the different constituent reactants of the gel. Crystals formed in a highly basic medium are generally small, of the order of a few micrometers to a few tens of micrometers. The more basic the medium, the smaller they are and they are then confined to the pores of the support employed.
The membranes obtained by the process of the invention are advantageously used in gas separation processes (gas or vapour permeation) or for separating liquids (pervaporation), optionally in filtration and electrophoresis processes. Separation processes that can be mentioned are:
separating linear and branched isomers of hydrocarbons containing 4 to 8 carbon atoms (C4 to C8);
separating paraffinic C6 and C7 isomers depending on the degree of branching (mono-, di-, tri-branched);
separating xylene isomers;
separating a methane/nitrogen mixture;
separating a hydrogen/hydrocarbon mixture (for example n-butane);
or separating a methane/carbon dioxide mixture.
The invention will be described in more detail with reference to non limiting examples 1 to 5. Example 2 is provided by way of comparison.
The entire disclosures of all applications, patents and publications, cited above or below, and of corresponding French application No. 01/01.746, filed Feb. 7, 2001, is hereby incorporated by reference.